Double-Network Physical Cross-Linking Strategy To Promote Bulk Mechanical and Surface Adhesive Properties of Hydrogels

A synergistic enhancement strategy for mechanical and conductive properties of hydrogels with dual ionically cross-linked κ-carrageenan/poly(sodium acrylate-co-acrylamide) network

Applying conductive hydrogels in electronic skin, health monitoring, and wearable devices has aroused great research interest. Yet, it remains a significant challenge to prepare conductive hydrogels simultaneously with superior mechanical, self-recovery, and conductivity performance. Herein, a dual ionically cross-linked double network (DN) hydrogel is fabricated based on K+ and Fe3+ ion cross-linked κ-carrageenan (κ-CG) and Fe3+ ion cross-linked poly(sodium acrylate-co-acrylamide) P(AANa-co-AM). Benefiting from the abundance of hydrogen bonds and metal coordination bonds, the conductive hydrogel has excellent mechanical properties (fracture strain up to 1420 %, fracture stress up to 2.30 MPa, and toughness up to 20.63 MJ/m3) and good self-recovery performance (the recovery rate of the toughness can reach 85 % after waiting for 1 h). Meanwhile, due to the introduction of dual metal ions of K+ and Fe3+, the ionic conductivity of conductive hydrogel is up to 1.42 S/m. Furthermore, the hydrogel strain sensor has good sensitivity with a gauge factor (GF) of 2.41 (0-100 %). It can be a wearable sensor that monitors different human motions, such as sit-ups. This work offers a new synergistic strategy for designing a hydrogel strain sensor with high mechanical, self-recovery, and conductive properties.

Flexible Artificial Tactility with Excellent Robustness and Temperature Tolerance Based on Organohydrogel Sensor Array for Robot Motion Detection and Object Shape Recognition

Hydrogel-based flexible artificial tactility is equipped to intelligent robots to mimic human mechanosensory perception. However, it remains a great challenge for hydrogel sensors to maintain flexibility and sensory performances during cyclic loadings at high or low temperatures due to water loss or freezing. Here, a flexible robot tactility is developed with high robustness based on organohydrogel sensor arrays with negligent hysteresis and temperature tolerance. Conductive polyaniline chains are interpenetrated through a poly(acrylamide-co-acrylic acid) network with glycerin/water mixture with interchain electrostatic interactions and hydrogen bonds, yielding a high dissipated energy of 1.58 MJ m-3, and ultralow hysteresis during 1000 cyclic loadings. Moreover, the binary solvent provides the gels with outstanding tolerance from -100 to 60 °C and the organohydrogel sensors remain flexible, fatigue resistant, conductive (0.27 S m-1), highly strain sensitive (GF of 3.88) and pressure sensitive (35.8 MPa-1). The organohydrogel sensor arrays are equipped on manipulator finger dorsa and pads to simultaneously monitor the finger motions and detect the pressure distribution exerted by grasped objects. A machine learning model is used to train the system to recognize the shape of grasped objects with 100% accuracy. The flexible robot tactility based on organohydrogels is promising for novel intelligent robots.

Physically Crosslinked Poly(methacrylic acid)/Gelatin Hydrogels with Excellent Fatigue Resistance and Shape Memory Properties

Hydrogels endure various dynamic stresses, demanding robust mechanical properties. Despite significant advancements, matching hydrogels' strength to biological tissues and plastics is often challenging without applying potentially harmful crosslinkers. Using hydrogen bonds as sacrificial bonds offers a promising strategy to produce tough, versatile hydrogels for biomedical and industrial applications. Poly(methacrylic acid) (PMA)/gelatin hydrogels were synthesized by thermally induced free-radical polymerization and crosslinked only by physical bonds, without adding any chemical crosslinker. The addition of gelatin increased the formation of hydrophobic domains in the structure of the hydrogels, which acted as permanent crosslinking points. The increase in PMA and gelatin contents generally led to a lower equilibrium water content (WC), higher thermal stability and better mechanical properties. The values of tensile strength and toughness reached up to 1.44 ± 0.17 MPa and 4.91 ± 0.51 MJ m-3, respectively, while the compressive modulus and strength reached up to 0.75 ± 0.06 MPa and 24.81 ± 5.85 MPa, respectively, with the WC being higher than 50 wt.%. The obtained values for compressive mechanical properties are comparable with super-strong hydrogels reported in the literature. In addition, hydrogels exhibited excellent fatigue resistance and biocompatibility, as well as great shape memory properties, which make them prominent candidates for a wide range of biomedical applications.

Spider-silk-inspired strong and tough hydrogel fibers with anti-freezing and water retention properties

Ideal hydrogel fibers with high toughness and environmental tolerance are indispensable for their long-term application in flexible electronics as actuating and sensing elements. However, current hydrogel fibers exhibit poor mechanical properties and environmental instability due to their intrinsically weak molecular (chain) interactions. Inspired by the multilevel adjustment of spider silk network structure by ions, bionic hydrogel fibers with elaborated ionic crosslinking and crystalline domains are constructed. Bionic hydrogel fibers show a toughness of 162.25 ± 21.99 megajoules per cubic meter, comparable to that of spider silks. The demonstrated bionic structural engineering strategy can be generalized to other polymers and inorganic salts for fabricating hydrogel fibers with broadly tunable mechanical properties. In addition, the introduction of inorganic salt/glycerol/water ternary solvent during constructing bionic structures endows hydrogel fibers with anti-freezing, water retention, and self-regeneration properties. This work provides ideas to fabricate hydrogel fibers with high mechanical properties and stability for flexible electronics.

Structure-Property Relationships of Granular Hybrid Hydrogels Formed through Polyelectrolyte Complexation

Hybrid hydrogels are hydrogels that exhibit heterogeneity in the network architecture by means of chemical composition and/or microstructure. The different types of interactions, together with structural heterogeneity, which can be created on different length scales, determine the mechanical properties of the final material to a large extent. In this work, the microstructure-mechanical property relationships for a hybrid hydrogel that contains both electrostatic and covalent interactions are investigated. The hybrid hydrogel is composed of a microphase-separated polyelectrolyte complex network (PEC) made of poly(4-styrenesulfonate) and poly(diallyldimethylammonium chloride) within a soft and elastic polyacrylamide hydrogel network. The system exhibits a granular structure, which is attributed to the liquid-liquid phase separation into complex coacervate droplets induced by the polymerization and the subsequent crowding effect of the polyacrylamide chains. The coacervate droplets are further hardened into PEC granules upon desalting the hydrogel. The structure formation is confirmed by a combination of electron microscopic imaging and molecular dynamics simulations. The interpenetration of both networks is shown to enhance the toughness of the resulting hydrogels due to the dissipative behavior of the PEC through the rupture of electrostatic interactions. Upon cyclic loading-unloading, the hydrogels show recovery of up to 80% of their original dissipative behavior in less than 300 s of rest with limited plasticity. The granular architecture and the tough and self-recoverable properties of the designed hybrid networks make them good candidates for applications, such as shape-memory materials, actuators, biological tissue mimics, and elastic substrates for soft sensors.

Advancements in gelatin-based hydrogel systems for biomedical applications: A state-of-the-art review

A gelatin-based hydrogel system is a stimulus-responsive, biocompatible, and biodegradable polymeric system with solid-like rheology that entangles moisture in its porous network that gradually protrudes to assemble a hierarchical crosslinked arrangement. The hydrolysis of collagen directs gelatin construction, which retains arginyl glycyl aspartic acid and matrix metalloproteinase-sensitive degeneration sites, further confining access to chemicals entangled within the gel (e.g., cell encapsulation), modulating the release of encapsulated payloads and providing mechanical signals to the adjoining cells. The utilization of various types of functional tunable biopolymers as scaffold materials in hydrogels has become highly attractive due to their higher porosity and mechanical ability; thus, higher loading of proteins, peptides, therapeutic molecules, etc., can be further modulated. Furthermore, a stimulus-mediated gelatin-based hydrogel with an impaired concentration of gellan demonstrated great shear thinning and self-recovering characteristics in biomedical and tissue engineering applications. Therefore, this contemporary review presents a concise version of the gelatin-based hydrogel as a conceivable biomaterial for various biomedical applications. In addition, the article has recapped the multiple sources of gelatin and their structural characteristics concerning stimulating hydrogel development and delivery approaches of therapeutic molecules (e.g., proteins, peptides, genes, drugs, etc.), existing challenges, and overcoming designs, particularly from drug delivery perspectives.

Highly Flexible and Self-Healing Supercapacitor Enabled by Physically Crosslinking Polymer Hydrogel Electrolyte

Preparation of flexible supercapacitors with excellent mechanical properties and self-healing properties is of great significance but still remains a challenge. A self-healable conductive hydrogel based on poly N-hydroxyethyl acrylamide (PHEAA) is fabricated as electrolyte for supercapacitors. The design of the physically cross-linked dual network, and rich hydrogen bonds endow the hydrogel with robust mechanical properties and strong self-healing ability. The hydrogel exhibited an excellent stretchability (723 %) and a high ionic conductivity (21.8 mS/cm). Specially, by in situ growth of electrode film, a non-laminated supercapacitor is obtained with flexibility and self-healing ability. Due to the non-laminated structure, the supercapacitor can work stably under bending and punching. The supercapacitor possessed an areal capacitance of 253.1 mF/cm2 and the capacitance retention was 80 % after five cutting-healing cycles. The pseudo-capacitance contribution of the supercapacitor after self-healing was discussed. It is noteworthy that the supercapacitor maintains the ability to power a clock after self-healing.

Continuous Spinning of High-Tough Hydrogel Fibers for Flexible Electronics by Using Regional Heterogeneous Polymerization

Hydrogel fibers have attracted substantial interest for application in flexible electronics due to their ionic conductivity, high specific surface area, and ease of constructing multidimensional structures. However, universal continuous spinning methods for hydrogel fibers are yet lacking. Based on the hydrophobic mold induced regional heterogeneous polymerization, a universal self-lubricating spinning (SLS) strategy for the continuous fabrication of hydrogel fibers from monomers is developed. The universality of the SLS strategy is demonstrated by the successful spinning of 10 vinyl monomer-based hydrogel fibers. Benefiting from the universality of the SLS strategy, the SLS strategy can be combined with pre-gel design and post-treatment toughening to prepare highly entangled polyacrylamide (PAM) and ionic crosslinked poly(acrylamide-co-acrylic acid)/Fe3+ (W-PAMAA/Fe3+ ) hydrogel fibers, respectively. In particular, the W-PAMAA/Fe3+ hydrogel fiber exhibited excellent mechanical properties (tensile stress > 4 MPa, tensile strain > 400%) even after 120 days of swelling in the pH of 3-9. Furthermore, owing to the excellent multi-faceted performance and one-dimensionality of W-PAMAA/Fe3+ hydrogel fibers, flexible sensors with different dimensions and functions can be constructed bottom-up, including the one-dimensional (1D) strain sensor, two-dimensional (2D) direction sensor, three-dimensional (3D) pressure sensor, and underwater communication sensor to present the great potential of hydrogel fibers in flexible electronics.

A Biodegradable, Adhesive, and Stretchable Hydrogel and Potential Applications for Allergic Rhinitis and Epistaxis

Bioadhesive hydrogels have attracted considerable attention as innovative materials in medical interventions and human-machine interface engineering. Despite significant advances in their application, it remains critical to develop adhesive hydrogels that meet the requirements for biocompatibility, biodegradability, long-term strong adhesion, and efficient drug delivery vehicles in moist conditions. A biocompatible, biodegradable, soft, and stretchable hydrogel made from a combination of a biopolymer (unmodified natural gelatin) and stretchable biodegradable poly(ethylene glycol) diacrylate is proposed to achieve durable and tough adhesion and explore its use for convenient and effective intranasal hemostasis and drug administration. Desirable hemostasis efficacy and enhanced therapeutic outcomes for allergic rhinitis are accomplished. Biodegradation enables the spontaneous removal of materials without causing secondary damage and minimizes medical waste. Preliminary trials on human subjects provide an essential foundation for practical applications. This work elucidates material strategies for biodegradable adhesive hydrogels, which are critical to achieving robust material interfaces and advanced drug delivery platforms for novel clinical treatments.

Stretchable Organohydrogel with Adhesion, Self-Healing, and Environment-Tolerance for Wearable Strain Sensors

Stretchable hydrogels as landmark soft materials have been efficiently utilized in the field of wearable sensing devices. However, these soft hydrogels mostly cannot integrate transparency, stretchability, adhesiveness, self-healing, and environmental adaptability into one system. Herein, a fully physically cross-linked poly(hydroxyethyl acrylamide)-gelatin dual-network organohydrogel is prepared in a phytic acid-glycerol binary solvent via a rapid ultraviolet light initiation. The introduction of gelatin as the second network endows the organohydrogel with desirable mechanical performance (high stretchability up to 1240%). The presence of phytic acid not only synergizes with glycerol to impart environment-tolerance to the organohydrogel (from -20 to 60 °C) but also increases the conductivity. Moreover, the organohydrogel demonstrates a durable adhesive performance toward diverse substrates, a high self-healing efficiency through heat treatment, and favorable optical transparency (transmittance of 90%). Furthermore, the organohydrogel achieves high sensitivity (gauge factor of 2.18 at 100% strain) and rapid response time (80 ms) and could detect both tiny (a low detection limit of 0.25% strain) and large deformations. Therefore, the assembled organohydrogel-based wearable sensors are capable of monitoring human joint motions, facial expression, and voice signals. This work proposes a facile route for multifunctional organohydrogel transducers and promises the practical application of flexible wearable electronics in complex scenarios.

Tough, Injectable Calcium Phosphate Cement Based Composite Hydrogels to Promote Osteogenesis

Osteoporosis is one of the most disabling consequences of aging, and osteoporotic fractures and a higher risk of subsequent fractures lead to substantial disability and deaths, indicating that both local fracture healing and early anti-osteoporosis therapy are of great significance. However, combining simple clinically approved materials to achieve good injection and subsequent molding and provide good mechanical support remains a challenge. To meet this challenge, bioinspired by natural bone components, we develop appropriate interactions between inorganic biological scaffolds and organic osteogenic molecules, achieving a tough hydrogel that is both firmly loaded with calcium phosphate cement (CPC) and injectable. Here, the inorganic component CPC composed of biomimetic bone composition and the organic precursor, incorporating gelatin methacryloyl (GelMA) and N-Hydroxyethyl acrylamide (HEAA), endow the system with fast polymerization and crosslinking through ultraviolet (UV) photo-initiation. The GelMA-poly (N-Hydroxyethyl acrylamide) (GelMA-PHEAA) chemical and physical network formed in situ enhances the mechanical performances and maintains the bioactive characteristics of CPC. This tough biomimetic hydrogel combined with bioactive CPC is a new promising candidate for a commercial clinical material to help patients to survive osteoporotic fracture.

Injectable Dopamine-Polysaccharide In Situ Composite Hydrogels with Enhanced Adhesiveness

Polysaccharide bio-adhesives used for non-invasive repair often show weak mechanical strength and tissue adhesion, even when covalently modified with dopamine (DA) from mussel proteins and its derivatives. Low cohesion of the polysaccharide adhesives and easy oxidation of DA may result in the low adhesion properties of the polysaccharide-DA adhesives. In this work, we aimed to prepare a series of injectable hydrogel adhesives to improve their cohesion and adhesion by in situ mixing DA with the polysaccharide without covalent modification. The injectable and rapid curing adhesives were prepared by mixing oxidized dextran (ODE) and chitosan (CS) through a Schiff base reaction in the presence (or absence) of DA. The gelation time of the adhesive was customized to be less than 20 s by controlling the amount of ODE, regardless of the amount of DA. Multi-cross-linked (MC) hydrogels were further prepared by adding cross-linking agents such as sodium periodate (NaIO₄) and ferric trichloride (FeCl₃), and their sol-gel transitions were easily adjusted by changing the amounts of the cross-linking agents. The MC-FeCl₃ hydrogel adhesive displayed good tissue adhesion with a lap shear adhesion strength of 345 kPa, which was 43 times that of fibrin glue. Results from Raman spectra, texture profile analyses, and atomic force microscopy images confirmed the enhanced adhesion induced by a higher cohesion of MC-FeCl₃, owing to the coordination of Fe³⁺ and DA and non-covalent and covalent bonds of DA. Moreover, the adhesives showed good biodegradability and biocompatibility. These results demonstrate that the injectable and sticky hydrogels with good adhesion are promising materials for tissue repair.

Bioinspired Injectable Self-Healing Hydrogel Sealant with Fault-Tolerant and Repeated Thermo-Responsive Adhesion for Sutureless Post-Wound-Closure and Wound Healing

Hydrogels with multifunctionalities, including sufficient bonding strength, injectability and self-healing capacity, responsive-adhesive ability, fault-tolerant and repeated tissue adhesion, are urgently demanded for invasive wound closure and wound healing. Motivated by the adhesive mechanism of mussel and brown algae, bioinspired dynamic bonds cross-linked multifunctional hydrogel adhesive is designed based on sodium alginate (SA), gelatin (GT) and protocatechualdehyde, with ferric ions added, for sutureless post-wound-closure. The dynamic hydrogel cross-linked through Schiff base bond, catechol-Fe coordinate bond and the strong interaction between GT with temperature-dependent phase transition and SA, endows the resulting hydrogel with sufficient mechanical and adhesive strength for efficient wound closure, injectability and self-healing capacity, and repeated closure of reopened wounds. Moreover, the temperature-dependent adhesive properties endowed mispositioning hydrogel to be removed/repositioned, which is conducive for the fault-tolerant adhesion of the hydrogel adhesives during surgery. Besides, the hydrogels present good biocompatibility, near-infrared-assisted photothermal antibacterial activity, antioxidation and repeated thermo-responsive reversible adhesion and good hemostatic effect. The in vivo incision closure evaluation demonstrated their capability to promote the post-wound-closure and wound healing of the incisions, indicating that the developed reversible adhesive hydrogel dressing could serve as versatile tissue sealant.

Manufacturing and post-engineering strategies of hydrogel actuators and sensors: From materials to interfaces

Living bodies are made of numerous bio-sensors and actuators for perceiving external stimuli and making movement. Hydrogels have been considered as ideal candidates for manufacturing bio-sensors and actuators because of their excellent biocompatibility, similar mechanical and electrical properties to that of living organs. The key point of manufacturing hydrogel sensors/actuators is that the materials should not only possess excellent mechanical and electrical properties but also form effective interfacial connections with various substrates. Traditional hydrogel normally shows high electrical resistance (~ MΩ•cm) with limited mechanical strength (<1 MPa), and it is prone to fatigue fracture during continuous loading-unloading cycles. Just like iron should be toughened and hardened into steel, manufacturing and post-treatment processes are necessary for modifying hydrogels. Besides, advanced design and manufacturing strategies can build effective interfaces between sensors/actuators and other substrates, thus enhancing the desired mechanical and electrical performances. Although various literatures have reviewed the manufacture or modification of hydrogels, the summary regarding the post-treatment strategies and the creation of effective electrical and mechanically sustainable interfaces are still lacking. This paper aims at providing an overview of the following topics: (i) the manufacturing and post-engineering treatment of hydrogel sensors and actuators; (ii) the processes of creating sensor(actuator)-substrate interfaces; (iii) the development and innovation of hydrogel manufacturing and interface creation. In the first section, the manufacturing processes and the principles for post-engineering treatments are discussed, and some typical examples are also presented. In the second section, the studies of interfaces between hydrogels and various substrates are reviewed. Lastly, we summarize the current manufacturing processes of hydrogels, and provide potential perspectives for hydrogel manufacturing and post-treatment methods.

Smart Antifreeze Hydrogels with Abundant Hydrogen Bonding for Conductive Flexible Sensors

Recently, flexible sensors based on conductive hydrogels have been widely used in human health monitoring, human movement detection and soft robotics due to their excellent flexibility, high water content, good biocompatibility. However, traditional conductive hydrogels tend to freeze and lose their flexibility at low temperature, which greatly limits their application in a low temperature environment. Herein, according to the mechanism that multi−hydrogen bonds can inhibit ice crystal formation by forming hydrogen bonds with water molecules, we used butanediol (BD) and N−hydroxyethyl acrylamide (HEAA) monomer with a multi−hydrogen bond structure to construct LiCl/p(HEAA−co−BD) conductive hydrogel with antifreeze property. The results indicated that the prepared LiCl/p(HEAA−co−BD) conductive hydrogel showed excellent antifreeze property with a low freeze point of −85.6 °C. Therefore, even at −40 °C, the hydrogel can still stretch up to 400% with a tensile stress of ~450 KPa. Moreover, the hydrogel exhibited repeatable adhesion property (~30 KPa), which was attributed to the existence of multiple hydrogen bonds. Furthermore, a simple flexible sensor was fabricated by using LiCl/p(HEAA−co−BD) conductive hydrogel to detect compression and stretching responses. The sensor had excellent sensitivity and could monitor human body movement.

Bioinspired Cationic-Aromatic Copolymer for Strong and Reversible Underwater Adhesion

Developing new underwater glue adhesives with robust and repeatable adhesion to various surfaces is promising and useful in marine life and medical treatments. In this work, we developed a novel glue based on a copolymer with a cation-co-aromatic sequence where the cationic units contain both catechol and positively charged sites. The glue consists of a crosslinked copolymer of poly(2-hydroxy-3-phenoxypropyl acrylate-co-3-(5-(3,4 dihydroxyphenyl)-4-oxo-3 N-pentyl)imidazolium) bromide in dimethyl sulfoxide. Solidification of the glue, triggered by contact with water, undergoes a coacervation stage and causes a drastic growth of its mechanical properties over time. The glue demonstrates fast-developing, strong, and repeatable underwater adhesion to different materials and can maintain its strength for a long time. The adhesion strength tends to increase with the surface energy of the substrate material, to a maximum value of 160 kPa found in plywood. Experiments conducted in aqueous media with different pH and ionic strengths, including physiological conditions and seawater, showed an even stronger adhesion than that evolved in deionized water. Thus, the developed glue is a promising candidate for use in marine life, tissue adhesives, and other freshwater and saline water applications.

Emerging Fabrication Strategies of Hydrogels and Its Applications

Recently, hydrogels have been investigated for the controlled release of bioactive molecules, such as for living cell encapsulation and matrices. Due to their remote controllability and quick response, hydrogels are widely used for various applications, including drug delivery. The rate and extent to which the drugs reach their targets are highly dependent on the carriers used in drug delivery systems; therefore the demand for biodegradable and intelligent carriers is progressively increasing. The biodegradable nature of hydrogel has created much interest for its use in drug delivery systems. The first part of this review focuses on emerging fabrication strategies of hydrogel, including physical and chemical cross-linking, as well as radiation cross-linking. The second part describes the applications of hydrogels in various fields, including drug delivery systems. In the end, an overview of the application of hydrogels prepared from several natural polymers in drug delivery is presented.

Structural aspects controlling the mechanical and biological properties of tough, double network hydrogels

Anticipating an increasing demand for hybrid double network (DN) hydrogels in biomedicine and biotechnology, this study evaluated the effects of each network on the mechanical and biological properties. Polyethylene glycol (PEG) (meth)acrylate hydrogels with varied monomer molecular weights and architectures (linear vs. 4-arm) were produced with and without an added ionically bonded alginate network and their mechanical properties were characterized using compression testing. The results showed that while some mechanical properties of PEG single network (SN) hydrogels decreased or changed negligibly with increasing molecular weight, the compressive modulus, strength, strain to failure, and toughness of DN hydrogels all significantly increased with increased PEG monomer molecular weight. At a fixed molecular weight (10 kDa), 4-arm PEG SN hydrogels exhibited better overall mechanical performance; however, this benefit was diminished for the corresponding DN hydrogels with comparable strength and toughness and lower strain to failure for the 4-arm case. Regardless of the PEG monomer structure, the alginate network made a relatively larger contribution to the overall DN mechanical properties when the covalent PEG network was looser with a larger mesh size (e.g., for larger monomer molecular weight and/or linear architecture) which presumably enabled more ionic crosslinking. Considering the biological performance, adipose derived stem cell cultures demonstrated monotonically increasing cell area and Yes-associated protein related mechanosensing with increasing amounts of alginate from 0 to 2 wt.%, demonstrating the possibility for using DN hydrogels in guiding musculoskeletal differentiation. These findings will be useful to design suitable hydrogels with controllable mechanical and biological properties for mechanically demanding applications. STATEMENT OF SIGNIFICANCE: Hydrogels are widely used in commercial applications, and recently developed hybrid double network hydrogels have enhanced strength and toughness that will enable further expansion into more mechanically demanding applications (e.g., medical implants, etc.). The significance of this work is that it uncovers some key principles regarding monomer molecular weight, architecture, and concentration for developing strong and tough hybrid double network hydrogels that would not be predicted from their single network counterparts or a linear combination of the two networks. Additionally, novel insight is given into the biological performance of hybrid double network hydrogels in the presence of adipose derived stem cell cultures which suggests new scope for using double network hydrogels in guiding musculoskeletal differentiation.

A bioinspired porous-designed hydrogel@polyurethane sponge piezoresistive sensor for human-machine interfacing

Conductive coating sponge piezoresistive pressure sensors are attracting much attention because of their simple production and convenient signal acquisition. However, manufacturing sponge-structure pressure-sensing materials with high compressibility and wide pressure detection ranges is difficult because of the instability of rigid and brittle conductive coatings at large strains. Herein, a tough conductive hydrogel@polyurethane (PU) sponge with a porous design is prepared via immersion of a polyurethane sponge in a low-cost and biocompatible polyvinyl alcohol (PVA)/glycerin (Gl)/sodium chloride (NaCl) solution. The sensor based on the hydrogel/elastomer sponge composite material exhibits a compressible range of 0-93%, a pressure detection range of 100 Pa-470.2 kPa, and 10 000-cycle stability (80% strain) because of the compressibility, flexibility, and toughness of the porous hydrogel coating. Benefiting from the resistance change mechanism of microporous compression, the sensor also exhibits a wide range of linear resistance changes, and the corresponding sensitivity and gauge factor (GF) are -0.083 kPa^-- (100 Pa-10.0 kPa) and -1.33 (1-60% strain), respectively. Based on its flexibility, compressibility, and wide-ranging linear resistance changes, the proposed sensor has huge potential application in human activity monitoring, electronic skin, and wearable electronic devices.

Molecular mechanism of abnormally large nonsoftening deformation in a tough hydrogel

Tough soft materials usually show strain softening and inelastic deformation. Here, we study the molecular mechanism of abnormally large nonsoftening, quasi-linear but inelastic deformation in tough hydrogels made of hyperconnective physical network and linear polymers as molecular glues to the network. The interplay of hyperconnectivity of network and effective load transfer by molecular glues prevents stress concentration, which is revealed by an affine deformation of the network to the bulk deformation up to sample failure. The suppression of local stress concentration and strain amplification plays a key role in avoiding necking or strain softening and endows the gels with a unique large nonsoftening, quasi-linear but inelastic deformation.

Strengthening gelatin hydrogels using the Hofmeister effect

A simple yet effective soaking treatment has been proposed to fabricate hydrogels with desirable mechanical properties, but the strengthening mechanism of hydrogels lacks an in-depth study. Here, we investigated the influence of kosmotropic citrate anion on the structure and properties of immersed gelatin hydrogels. The obtained hydrogels possessed the properties of high strength, modulus and toughness simultaneously. The dehydration of hydrogels facilitated the interactions among gelatin molecules, resulting in the formation of helix structures. Both the content and length of the triple helices increase with an increase in citrate concentration, which in turn contributes to the strengthening of hydrogels. The excellent mechanical performances of these hydrogels may open up new applications for protein materials.

Tannic Acid-Silver Dual Catalysis Induced Rapid Polymerization of Conductive Hydrogel Sensors with Excellent Stretchability, Self-Adhesion, and Strain-Sensitivity Properties

The application of conductive hydrogels in intelligent biomimetic electronics is a hot topic in recent years, but it is still a great challenge to develop the conductive hydrogels through a rapid fabrication process at ambient temperature. In this work, a versatile poly(acrylamide) @cellulose nanocrystal/tannic acid-silver nanocomposite (NC) hydrogel integrated with excellent stretchability, repeatable self-adhesion, high strain sensitivity, and antibacterial property, was synthesized via radical polymerization within 30 s at ambient temperature. Notably, this rapid polymerization was realized through a tannic acid-silver (TA-Ag) mediated dynamic catalysis system that was capable of activating ammonium persulfate and then initiated the free-radical polymerization of the acrylamide monomer. Benefiting from the incorporation of TA-Ag metal ion nanocomplexes and cellulose nanocrystals, which acted as dynamic connecting bridges by hydrogen bonds to efficiently dissipate energy, the obtained NC hydrogels exhibited prominent tensile strain (up to 4000%), flexibility, self-recovery, and antifatigue properties. In addition, the hydrogels showed repeatable adhesiveness to different substrates (e.g., glass, wood, bone, metal, and skin) and significant antibacterial properties, which were merits for the hydrogels to be assembled into a flexible epidermal sensor for long-term human-machine interfacial contact without concerns about the use of external adhesive tapes and bacterial breeding. Moreover, the remarkable conductivity (σ ∼ 5.6 ms cm^-1) and strain sensitivity (gauge factor = 1.02) allowed the flexible epidermal sensors to monitor various human motions in real time, including huge movement of deformations (e.g., wrist, elbow, neck, shoulder) and subtle motions. It is envisioned that this work would provide a promising strategy for the rapid preparation of conductive hydrogels in the application of flexible electronic skin, biomedical devices, and soft robotics.

Novel Salt-Responsive SiO2@Cellulose Membranes Promote Continuous Gradient and Adjustable Transport Efficiency

Continuously growing interest in the controlled and tunable transport or separation of target molecules has attracted more attention recently. However, traditional "on-off" stimuli-responsive membranes are limited to nongradient feedback, which manifests as filtration efficiency that cannot be increased or decreased gradually along with the different stimuli conditions; indeed, only the transformation of on/off state is visible. Herein, we design and fabricate a series of robust salt-responsive SiO₂@cellulose membranes (SRMs) by simply combining salt-responsive poly[3-(dimethyl(4-vinylbenzyl)ammonium)propyl sulfonate] (polyDVBAPS)-modified SiO₂ nanoparticles and cellulose membranes under negative-pressure filtering. The antipolyelectrolyte effect induces stretch/shrinkage of polyDVBAPS chains inside the channels and facilities the directional aperture size and surface wettability variation, greatly enhancing the variability of interfacial transport and separation efficiency. Due to the linear salt-responsive feedback mechanism, the optimal SRMs achieve highly efficient target macromolecule separation (>75%) and rapid oil/saline separation (>97%) with a continuous gradient and adjustable permeability, instead of simply an "on-off" switch. The salt-responsive factors (SiO₂-polyDVBAPS) could be reversibly separated or self-assembled to membrane substrates; thus, SRMs achieved unprecedented repeatability and reusability even after long-term cyclic testing, which exceeds those of currently reported membranes. Such SRMs possess simultaneously a superfast responsive time, a controllable gradient permeability, a high gating ratio, and an excellent reusability, making our strategy a potentially exciting approach for efficient osmotic transportation and target molecule separation in a more controllable manner.

A Review of Conductive Hydrogel Used in Flexible Strain Sensor

Hydrogels, as classic soft materials, are important materials for tissue engineering and biosensing with unique properties, such as good biocompatibility, high stretchability, strong adhesion, excellent self-healing, and self-recovery. Conductive hydrogels possess the additional property of conductivity, which endows them with advanced applications in actuating devices, biomedicine, and sensing. In this review, we provide an overview of the recent development of conductive hydrogels in the field of strain sensors, with particular focus on the types of conductive fillers, including ionic conductors, conducting nanomaterials, and conductive polymers. The synthetic methods of such conductive hydrogel materials and their physical and chemical properties are highlighted. At last, challenges and future perspectives of conductive hydrogels applied in flexible strain sensors are discussed.